京 都 大 学 防 災 研 究 所 年 報 第 50 号 C 平 成 19 年 4 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 50 C, 2007

Investigation of the Effects of Urban Heating on the Heavy Rainfall Event by a Cloud
Resolving Model CReSiBUC
Shuichi IKEBUCHI, Kenji TANAKA, Yotaro ITO*, Qoosaku MOTEKI**,
Kazuyoshi SOUMA* and Kazuaki YOROZU** Graduate School of Engineering, Kyoto University** Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Synopsis
In this study, effects of urban heating on the development of the heavy rainfall on 21 July
1999 in Tokyo (the Nerima heavy rainfall) were investigated by a coupled model of a cloud
resolving model CReSS and a precise land surface model SiBUC (CReSiBUC). Four numericalsimulations were carried out for this rainfall event. The first simulation had realistic land cover,second one had imaginary land cover (urban area was changed into paddy field), the third one had
imaginary land cover (urban area was enlarged) and forth one had realistic land cover andimaginary anthropogenic heat. From those simulations, it was found that changes of distribution ofurban and anthropogenic heat amount greatly affected on the positions and amounts of rainfall.
Keywords: urban heat island, heavy rainfall, cloud resolving model, CReSiBUC

1.

Introduction

In late years, localized heavy rainfall over urbanizedarea with a short time duration has become problem.
Frequent occurrence of localized heavy rainfall overurbanized area and its connection with heat island
phenomena have been pointed out, but a clear answer is
not obtained. As for the connection of heat island andrainfall, Shepherd (2003) pointed out that heat island
associated with artificial influence causes rainfall andincreases the frequency of a thunderbolt in a big city
along the seashore such as Houston in Texas. Kobayashi(2003) investigated the generation and the development
processes of cumulonimbus clouds in an urban area
using weather radar data. They pointed out that theprocesses of the cumulonimbus clouds in the urban area
were different from those in mountainous regions. Fujibe(2003) suggested that the “heat island” affected thefrequency of strong rainfall events in the Tokyometropolitan area. These suggestions with observational

studies were encouraged by some numerical studies (e.g.
Gero and Pitman, 2006). However, the urban effects on
rainfall events have never been discussed very well inJapan.
In this study, the effects of urban heating on thedevelopment of the strong cumulonimbus clouds wereinvestigated by a coupled model of a non-hydrostatic
meteorological model CReSS (Cloud Resolving StormSimulator) developed at Nagoya Univ., Japan and a
precise land surface model SiBUC (Simple Biospheremodel including Urban Canopy) developed at Kyoto
Univ., Japan. This model (CReSiBUC) is able to include
the existence of urban area and anthropogenic heatrelease as a part of land surface processes. To study theeffects of urban heating on rainfall, sensitivity tests for
distribution of existence of urban area and anthropogenic
heat in urban are conducted by the CReSiBUC. A case of
the heavy rainfall on 21 July 1999 in Tokyo (the Nerima
heavy rainfall) is selected for the sensitivity tests. Thisrainfall event was a localized one occurred in a highly－ 105 －Fig.1 (a) Averaged surface temperature for 12-14 JST with the AMeDAS, and (b) accumulated rainfall for 15-18
JST with the Rader-AMeDAS on 21 July 1999. TMA is surrounded by the red line.
urbanized area (Nerima-ward, Tokyo, Japan), and therainfall amount was 130 mm within 3 hours (15-18 JST).
This is a typical and suitable case for studying the urban
effects on rainfall.

observational evidences, it is suggested that the Nerima
heavy rainfall was strongly affected by the existence ofurban area.
3.

2.

Observational Characteristics of theNerima Heavy RainfallAveraged surface temperature and wind by theAMeDAS (Automated Meteorological Data Acquisition
System) for 12-14 JST (the times to consider theatmosphere ground before several hours of rainfalloutbreak) and accumulated rainfall by theRadar-AMeDAS for 15-18 JST are shown in Fig.1. In
this study, a domain where the ratio of urban area per 5km square unit is more than 30% is defined as "Tokyo
Metropolitan Area (TMA)" in real land use distribution
for convenience. Before the generation of the Nerima
heavy rainfall, the weather was sunny around Tokyo
district, and the feature of the “heat island” was clearlyshown in the distribution of the surface temperature. Thetemperature above 33 degree C was observed in TMA
and the temperature outside of TMA was below 31
degree C (Fig. 1(a)). The surface winds in and around
TMA tended to converge into the center of TMA
The strong rainfall exceeding 50 mm associated with
the Nerima heavy rainfall was locally induced in the areaof about 50 km square (Fig. 1(b)). The position of thestrong rainfall corresponded to that of the area of hightemperature and convergence zone. From theseModel Description of CReSiBUC

The CReSiBUC is a coupled model of the CReSS
and the SiBUC. The CReSS is a non-hydrostatic
meteorological model developed in Hydrospheric
Atmospheric Research Center, Nagoya University(Tsuboki and Sakakibara, 2001). The CReSS has two
significant advantages. One is a high readability of thesource code so that the CReSS is easy to couple withmodels for other processes. The other is a highcalculation efficiency that satisfies a condition to use the“Earth Simulator,” which is one of the fastest supercomputers in the world. Using the CReSS, atmosphericsimulations for the broader domain with the higher
resolution are possible.
The SiBUC is a land surface processes model
developed in Water Resources Research Center, DisasterPrevention Research Institute, Kyoto University (Tanaka,2004). The SiBUC calculates surface fluxes and relatedhydrological quantities considering detailed processes. In
the normal CReSS, the land surface processes arecalculated with empirical constant values of albedo,
evaporation efficiency, and roughness for simplifying
calculations. To the contrary, the SiBUC calculates
budgets of radiation, heat, water, and momentum whilechanging parameters for the surface condition. For－ 106 －example, for an urban area in the SiBUC, uneven
distribution of roof height is considered on the basis of
the urban canyon concept. Additionally, the SiBUC
adopts a “mosaic” approach. In case that a horizontal
grid of the CReSS is including a number of various land
use categories, values on each category are calculated
and a grid averaged surface fluxes are returned to theCReSS. This mosaic scheme is an effective facility
especially for the domain where multiple artificial
landuse categories of urban, paddy, etc. are mixed as like
Japan.
4.

(CTL)

Simulation Design

In the simulations with the CReSiBUC, precipitation
physics of mixed-phase processes are used. The
horizontal grid size is 5 km, and the vertical gridcontains 45 levels with variable grid intervals (Ӡz=100
m near the surface and Ӡz=500 m at the top level). The
horizontal domain has 100100 grid points, and big andsmall time steps of Ӡt=10 seconds and Ӡt=5 seconds
are used.
The initial and lateral boundary data were provided
from output produced by the Regional Spectral Model(RSM: a hydrostatic model used operationally in Japan
Meteorological Agency, referred in Segami et al., 1987).
The CReSiBUC simulation was one-way nested within
the RSM started at 0900 JST on 21 July 1999. The
integration time of the CReSiBUC simulation was 12hours from 0900 JST on 21 July 1999, and lateralboundary data were given every hour. The simulation
domains for the RSM and the CReSiBUC are shown inFig.2.

Fig.2
Model domains of JMA-RSM, andCReSiBUC.

(PDY)

(UBN)
Fig.3 Land-use distribution given to the CTL, PDY,and UBN simulation. Fraction of urban area andpaddy field are shown in the left and right side,respectively.
To study the effects of the existence of urban heatingon the generation of the Nerima heavy rainfall,
simulations called CTL, PDY, and UBN (abbreviationsof control, paddy, and urban, respectively) are conductedusing the following three patterns of the land-use
distributions. Fig.3 shows that the first one (CTL) is theactual distribution, the second one (PDY) is thedistribution in which urban area is replaced with paddy
field, the third one (UBN) is the distribution in whichfraction of urban in TMA is enhanced. In the UBN
simulation, all land area (except water body) in TMA istreated as urban. In addition, to consider the effects of－ 107 －(a) Rainfall(b) Temperature(c) ConvergenceFig.4 Simulated results with the CTL of (a) accumulated rainfall for 1500-1800 JST, (b) averaged temperature at
100 m for 1200-1400 JST, (c) averaged convergence accumulated in the 100-2000 m-layer for 1600-1800 JST. TMA
where the fraction of urban area is over 30 % is surrounded by red line.

(a) Rainfall(b) Temperature-5(d) Temp. difference(c) Convergence-1 -0.5 -0.1 0.1 0.5

15(e) Conv. difference

Fig.5 Simulated results with the PDY of (a) accumulated rainfall for 15-18 JST, (b) averaged temperature at 0.1 km
for 12-14 JST, (c) averaged convergence accumulated in the 0.1-2.0 km-layer for 16-18 JST, (d) averaged
temperature difference between the PDY and the CTL, and (e) averaged convergence between the PDY and theCTL.

－ 108 －anthropogenic heat in urban, a simulation that theanthropogenic heat of 400 Wm-2 is given (hereaftercalled AHD) is conducted. The anthropogenic heat is an
artificial source due to cooling equipments, cars, etc.
According to the distribution data of urban
anthropogenic heat in Tokyo (Ichinose et al., 1994), aheat amount of about 300-400 Wm-2 is discharged inTokyo during a summer season. Although the realistic
anthropogenic heat amount is changed in time, theconstant amount of 400 Wm-2 is assumed in the AHD
simulation.
5.

Results and Discussions

Comparing to the observation results in Fig.1, themain features of the Nerima Heavy Rainfall was well
produced by the CTL simulation as shown in Fig.4. A
local strong rainfall over 35 mm is located in TMA,
although its position is slightly shifted in the southwest
from that of the actual the Nerima Heavy Rainfall. This
shift could result from accuracy of the initial and lateral(a) Rainfall

boundary data from the RSM. A high temperature area(above 30 degree C) is seen in the distribution of lowest
level (100 m) temperature as shown in the actual
observation. The position of convergence in the lower
level atmosphere (100-2000 m) exceeding 10×10-4 s-1 iswell corresponded to the high temperature area.
The results of the PDY simulation and theirdifference from the CTL simulation are shown in Fig.5.
Comparing to the CTL, a strong rainfall area is shifted
further west. In the PDY, a high temperature area inTMA is not seen (Fig.5 (b)), that is, the "heat island"
phenomenon is not occurred. Around the area where thetemperature decrease is exceeding 2 degree C, winds arechanged to the direction to the outside of TMA (Fig.5(d)). No remarkable convergence zone is seen in TMA(Fig.5 (c)). Comparing to the CTL, the convergence inTMA is decreased by 5×10-4 s-1, meanwhile theconvergence in a mountainous region in the west isfound to be increased (Fig.5 (e)).
The results of the UBN simulation and theirdifference from the CTL simulation are shown in Fig.6.

(b) Temperature(d) Temp. difference(c) Convergence(e) Conv. Difference

Fig.6 The same as Fig.5, but given to the UBN simulation.

－ 109 －(a) Rainfall(b) Temperature(d) Temp. difference(c) Convergence(e) Conv. Difference

Fig.7 The same as Fig.5, but given to the AHD simulation.
Comparing to the CTL, a strong rainfall area is shifted
further east. In the UBN, a high temperature area (above30 degree C) in TMA is extended, that is, the feature ofthe "heat island" in the UBN is more significant than thatin the CTL (Fig.6 (b)). Around the area where thetemperature increase is exceeding 1 degree C, the winds
are changed to the direction toward TMA (Fig.6 (d)). Aremarkable convergence zone is seen in TMA as in theCTL (Fig.6 (c)). Comparing to the CTL, the convergencein TMA is increased by over 5×10-4 s-1.
It can be explained that the position of theprecipitation clouds formation is shifted due to the factthat the surface heating is changed resulting from theexistence and concentration of urban area. From theseresults of the PDY and the UBN, it is considered that theexistence of urban changes the low-level convergencethrough the effects of the "heat island," and significantlyaffects the position of the formation of cumulonimbus
clouds to cause a heavy rainfall.
The results of the ADH simulation and theirdifference from the CTL simulation are shown in Fig.7.

Comparing to the CTL, a strong rainfall area is extended
in the west-east direction. An increase of rainfall amountof 10 mm in the maximum was seen in TMA In theAHD, a high temperature area (above 32 degree C) in
TMA is seen, and the feature of the "heat island" in theAHD is more significant than that in the UBN (Fig.7 (b)).
Around the area where the temperature increase isexceeding 2 degree C, winds are changed to the direction
toward TMA (Fig.7 (d)). A remarkable convergencezone is seen in TMA as in the CTL (Fig.7 (c)).
Comparing to the UBN, the area of the increase of
10×10-4 s-1 from the CTL is extended (Fig.7 (e)).
This result cautions about the following scenario.
Buildings will become higher and higher, andanthropogenic heat per unit area associated with coolingequipments will increase more and more. It is suggested
that growing of such kind of human-induced heating
affects the intensity and frequency of heavy rainfall
disasters in urban area.

－ 110 －6.

Fujibe, F. (2003): A review of effects of heat island onrainfall -Summer convective rainfall events-, Bull. Jour.
of Meteor. Soc. of Japan Tenki, 51, 109-115, (inJapanese).
Gero, A. F., Pitman, A. J. (2006): The Impact of Land
Cover Change on a Simulated Storm Event in theSydney Basin, Journal of Applied Meteorology andClimatology. Vol. 45, 283-300.
Ichinose, T., K. Hanaki, T. Matsuo (2003): Analyses ofstructure of temporal and spatial distribution of urban
anthropogenic heat based on a fine-mesh geographic
information, Jour. of Japan Soc. of Civil Engineering,31, 267-273 (in Japanese).
Kobayashi, K. (2003): Effects of heat island on rainfall,Bull. Jour. of Meteor. Soc. of Japan Tenki, 51, 115-117(in Japanese),.
Marshall J. Shepherd and Steven J. Burian (2003):
Detection of Urban-Induced Rainfall Anomallies in aMajor Coastal City, Earth Interactions. Vol. 7, No. 4,1-17.
Tanaka, K. (2004): Development of the new land surface
scheme SiBUC commonly applicable to basin water
management and numerical weather prediction model,
doctoral dissertation, Kyoto Univ.
Tsuboki, K. and A. Sakakibara (2001): CReSS User’sGuide Ver.2, 256pp.

Conclusions

In this study, effects of urban heating on thedevelopment of the heavy rainfall on 21 July 1999 inTokyo (the Nerima heavy rainfall) were investigated by acoupled model of a cloud resolving model CReSS and a
precise land surface model SiBUC (CReSiBUC).
Changing the land cover condition, four numericalsimulations were carried out for this rainfall event. From
those simulations, it was found that changes of
distribution of urban area and anthropogenic heat amountgreatly affected on the positions and amounts of rainfall.
Acknowledgements
In this study, we used cloud resolving model CReSS
that was developed by Kazuhisa Tsuboki (Nagoya Univ.)and Atsushi Sakakibara (Chuden CTI Co, Ltd). We show
gratitude to here. In addition, we express sincere
appreciation for Dr. Teruyuki Kato. He set up theanalysis data of RSM. This study was performed as apart of DPRI, Kyoto Univ. COE.

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